In a local area network (LAN) a user, such as a portable computer equipped with communication capability, gains access to the LAN via a physical connection in order to communicate with remote facilities or use shared resources, such as file servers, print servers, etc. In a stationary mode of operation, all users are static and each user gains access to the network via a fixed homing point. However, in a mobile environment users are free to change their physical location and cannot be restricted to gain access to the network only through one of several homing points attached to the LAN. In a mobile environment the homing points are fixed base stations that communicate with the mobile users through a wireless link. Examples of wireless links include radio frequency (RF) links, microwave links and infrared (IR) links.
Of interest herein are wireless communication networks having overlapping coverage areas or cells wherein the same frequencies are employed for the uplink and the downlink, respectively, within each coverage area. The use of the same frequencies in overlapping communication cells has the advantage that it eliminates a requirement that the mobile users switch frequencies when migrating from one cell to another. An additional advantage is that the effective wireless bandwidth can be increased by spatial reuse of frequencies in non-overlapping cells. This technique is called Frequency Reuse.
However, the transmission and reception of messages in a cell of a multicell network, of the type that employs identical communication frequencies in different cells, requires control of interference between users. This interference may occur from several sources including uplink transmissions from mobile users that lie in overlapping areas between adjacent cells and downlink transmissions from base stations if their overlapping cell areas contain one or more mobile users.
The following U.S. Patents and articles are made of record for teaching various aspects of mobile communication.
The following two U.S. Patents show communication systems having overlapping coverage areas. U.S. Pat. No. 4,597,105, Jun. 24, 1986, entitled "Data Communications System having Overlapping Receiver coverage Zones" to Freeburg and U.S. Pat. No. 4,881,271, issued Nov. 14, 1989, entitled "Portable Wireless Communication Systems" to Yamauchi et al. Yamauchi et al. provide for a hand-off of a subscriber station from one base station to another by the base station continually monitoring the signal strength of the subscriber station.
The following U.S. patents teach various aspects of wireless communication networks.
In U.S. Pat. No. 4,792,946, issued Dec. 20, 1988, entitled "Wireless Local Area Network for Use in Neighborhoods" S. Mayo describes a local area network that includes transceiver stations serially coupled together in a loop.
In U.S. Pat. No. 4,777,633, issued Oct. 11, 1988, entitled "Base Station for Wireless Digital Telephone System" Fletcher et al. describe a base station that communicates with subscriber stations by employing a slotted communications protocol.
In U.S. Pat. No. 4,730,310, issued Mar. 8, 1988, entitled "Terrestrial Communication System" Acampora et al. describe a communications system that employs spot beams, TDMA and frequency reuse to provide communication between a base station and remote stations.
In U.S. Pat. No. 4,655,519, issued May 12, 1987, entitled "Wireless Computer Modem" Kirchner et al. disclose a wireless modem for transferring data in a computer local area network.
In U.S. Pat. No. 4,639,914, issued Jan. 27, 1987, entitled "Wireless PBX/LAN System with Optimum Combining" Winters discloses a wireless LAN system that employs adaptive signal processing to dynamically reassign a user from one channel to another.
In U.S. Pat. No. 4,837,858, issued Jun. 6, 1989, entitled "Subscriber Unit for a Trunked Voice/Data Communication System" Ablay et al. disclose a trunked voice/data subscriber that operates in either a voice mode or one of three data modes.
In U.S. Pat. No. 4,852,122, issued Jul. 25, 1989, entitled "Modem Suited for Wireless Communication Channel Use" Nelson et al. disclose a wireless communication system and, specifically, a modem that communicates digital data with data terminal equipment.
In U.S. Pat. No. 4,926,495, issued May 15, 1990 entitled "Computer Aided Dispatch System" Comroe et al. disclose a computer aided dispatch system that includes a master file node and a plurality of user nodes. The master file node maintains a record for each subscriber and automatically transmits an updated record to each dispatcher attached to a subgroup in which the subscriber operates.
In U.S. Pat. No. 4,456,793, issued Jun. 26, 1984, W. E. Baker et al. describe a cordless telephone system having infrared wireless links between handsets and transponders. The transponders are wired to subsystem controllers which are in turn wired to a system controller. The central controller polls the cordless stations every 100 milliseconds to detect cordless station locations and to identify "missing" cordless stations.
In U.S. Pat. No. 4,807,222, issued Feb. 21, 1989 N. Amitay describes a LAN wherein users communicate with RF or IR signals with an assigned Regional Bus Interface Unit (RBIU). Protocols such as CSMA/CD and slotted ALOHA are employed in communicating with the RBIUs.
In commonly assigned U.S. Pat. No. 4,402,090, issued Aug. 30, 1983, F. Gfeller et al. describe an infrared communication system that operates between a plurality of satellite stations and a plurality of terminal stations. A host computer communicates with the terminal stations via a cluster controller and the satellite stations, which may be ceiling mounted. Communication with the terminal stations is not interrupted even during movement of the terminal stations.
In U.S. Pat. No. 4,573,206, issued Feb. 25, 1986, Granel et al. describe a cellular radio system where the control channel assignment to mobile stations is done by the base station distributing information such as number of groups and group codes.
In U.S. Pat. No. 4,551,852, issued Nov. 5, 1985, Granel et al. describe a cellular radio system for spreading the volume of traffic over different control channels.
In U.S. Pat. No. 4,789,983, issued Dec. 6, 1988, Acampora et al. describe a multiple access protocol design for an indoor radio network. The proposed centralized approach uses a modified polling protocol to achieve communications between remote stations.
In IBM Technical Disclosure Bulletin, Vol. 20, No. 7, December 1977 F. Closs et al. describe the use of both line-of-sight and diffuse transmission of infrared signals for wireless communications between a ceiling-based controller and a plurality of terminals.
In IBM Technical Disclosure Bulletin, Vol. 24, No. 8, page 4043, January 1982 F. Gfeller describes general control principles of an infrared wireless network incorporating multiple ceiling mounted transponders that couple a host/controller to multiple terminal stations. Access to the uplink channel is controlled by a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) method.
In U.S. patent application Ser. No. 07/605,291 filed Oct. 29, 1990, entitled "Scheduling Methods for Efficient Frequency Reuse in a Multi-Cell Wireless Network Served By A Wired Local Area Network" by K.S. Natarajan, which is assigned to the assignee of this invention, a method includes a first step of receiving a message from a wired network with at least one base station. The message is a message type that authorizes a base station or base stations to transmit on a wireless network. A second step of the method is accomplished in a response to the received first message, and initiates wireless communications with any mobile communication units that are located within a communication cell served by the at least one base station. In one embodiment the message circulates around a token ring and activates base stations in turn. In another embodiment disjoint groups of base stations are predetermined by graphical techniques, including graph coloring methods. The predetermined group information is maintained by a network controller that thereafter selectively activates the base stations within the different groups.
What is not taught by this prior art, and what is thus an object of the invention to provide, are communication methodologies that realize an efficient scheduling and frequency reuse in a wireless communications network that is served by a wired network. This is accomplished through the use of a centralized method that makes scheduling decisions in a state-dependent manner.